A real knee exhibits two types of flexing actions during locomotion, depending on whether it is unloaded or loaded. Locomotion comprises a swing phase, when the leg is lifted off the ground, and a stance phase, when the foot contacts the ground. In the swing phase, the knee is unloaded and is capable of almost free flexion, being able to swing as much as 60.degree. to 70.degree.. In the stance phase, the knee is loaded, and the knee's flexion ability is limited to no more than about 10.degree. to 20.degree..
A graph of the vertical component of the ground reaction force, F.sub.y, on a person's leg during the stance phase of locomotion is illustrated in FIG. 6a, and the approximate orientation of the leg during the stance phase is illustrated in FIG. 6b. As the leg initially contacts the ground, F.sub.y increases from 0 to a first peak. The knee is only slightly flexed, the angle of flexion being braked or limited during the stance phase of a real knee. The magnitude of F.sub.y at the first peak is slightly greater than the patient's weight, W, during walking, as seen in FIG. 6a. F.sub.y can be approximately three times the patient's weight during running. F.sub.y decreases slightly during the middle of the stance phase F.sub.y then increases to a second peak, approximately of the same magnitude as the first peak, and the knee is flexed slightly in preparation for the greater flexion during the swing phase, which occurs when F.sub.y decreases to 0 and the foot lifts off the ground.
It has been a challenge to create a prosthetic knee unit capable of this dual action: almost free flexion within 60.degree.-70.degree. during swing phase and flexion within 10.degree.-20.degree. during stance phase. Preliminary feasibility studies, by L. D. Fisher and G. W. Judge, indicate that prosthetic knee stance phase flexion results in shock absorption and reduces the vertical motion of the center of mass of the body. A knee design based on this work has a mono-centric, load-activated brake joint with a torsional elastic coupling between the main axis and the brake drum. This knee provides a stance phase flexion similar to that of a sound gait, but a resistive element demonstrates dissipative behavior, which does not fully resemble the elastic behavior of the human knee. Another disadvantage is that the stability of the knee is reduced, while the flexion angle during the stance phase increases.
A dual-action artificial knee achieved with a single-axis design is disclosed by A. P. Kuzekin et al. However, this knee may be unstable and potentially dangerous in actual use, because triggering of the action is controlled by the prosthesis's foot position.
Increased stability has been achieved in polycentric knee mechanisms using four-bar and five-bar-linkages, disclosed in Van de Veen, P. G., Wagner, H., Krieger, W., "A Polycentric Stance Phase Flexion Prosthetic Knee Mechanism," Proc. of the Seventh World Congress of ISPO, Chicago, Ill., Jun. 28-Jul. 3, 1992, p. 175. But these as well as all other multi-axis knees comprise redundant links, which are difficult to manufacture and maintain.
A prosthetic foot has been designed using non-congruent rolling joint surfaces combined with and held together by linear elastic springs which more closely mimic actual joint motion, as disclosed in U.S.S.R. Inventor's Certificate No. 820,822, of M. R. Pitkin and I. A. Mendelevich, entitled "Artificial Foot." A "rolling joint" approach has also been used in U.S. patent application Ser. No. 07/947,919, of the present inventor entitled "Artificial Foot and Ankle," the disclosure of which is incorporated herein by reference. The basic mechanical principles used in the artificial foot and ankle are discussed with reference to FIGS. 1 and 2.
A rolling joint of an ankle or toe between an upper bone 2, e.g., a tibia, and a lower bone 4, e.g., a talus, is shown in FIG. 1. The joint is joined by an elastic tie or spring 6, illustrated in the initial, unloaded or neutral, position, by the line segment OM.sub.0 having a length l.sub.0.
In the initial state, shown by a solid line, the upper bone's articular surface contacts the lower bones's articular surface at point K.sub.0. Due to a clockwise rotation of the upper bone, an arc with a center C.sub.0 and radius r=C.sub.0 K.sub.0 rolls along the surface of the bottom bone's articular surface. Contact between the bones occurs at point K.sub.1.
The rotation causes the elastic tie at position M.sub.0 to shift to position M.sub.1 along a cycloidal path, which causes a tension force T=.mu..DELTA.l in the spring, now at OM.sub.1, where .DELTA.l=l.sub.1 -l.sub.0 and l.sub.1 =OM.sub.1. The instantaneous arm L(.alpha.)=K.sub.1 N of the force T increases as the angle .alpha. of rotation of the upper bone increases; L(.alpha.) does not remain equal to K.sub.0 O as it would in a simple hinge rotatable about a fixed point.
Since the moment M of the force T is determined as M=TL(.alpha.), we obtain after calculation of L(.alpha.): ##EQU1## where the value of .alpha..sub.0 corresponds to the initial position of the joint before the deflection affecting the tie 6, at OM.sub.0.
As analysis shows, the diagram of M will always be convex downward, as seen in FIG. 2a. FIG. 2b provides a graphical illustration for l.sub.0 =0.01 m, .mu.=3.times.10.sup.4 N/m. Three curves are shown, for r=0.05 m, r=0.10 m and r=0.20 m.
As seen in FIG. 2, when the angular deflection .alpha. of a joint approaches the natural limit of its range of motion (B+,B-), as motion continues and .vertline..alpha..vertline..gtoreq..alpha..sub.0, the moment M of ligament resistance increases rapidly and nonlinearly. This saturation, or increase in resistance, of the cam structure occurs because the point of contact of the elements during flexing has a horizontal component of the relative rolling motion between the contacting surfaces. The rolling joint may also be considered to be a succession of hinged two-element systems, the arm of each of which is greater than the arm of the previous system.
Even though the zones or areas at which the elastic ties are attached in this foot and ankle unit are movable with respect to each other for better control over the tension in the ties, this design cannot be directly applied to an artificial knee, because of the dual character of knee flexion: relatively small amortization flexion during stance phase and relatively large flexion during swing phase. Thus, a need still exists for a knee design which provides the dual action observed in a normal gait.